Method for hardening chromiumnickel stainless steel



IVIETHOD FOR HARDENING CHROMIUM- NICKEL STAINLESS STEEL D Cameron Perry, Middletown, Ohio, assignor to Armco Steel Corporation, a corporation of Ohio N Drawing. Filed July 31, 1957, Ser. No. 675,282

Claims. (Cl. 148- -1 25) My invention relates to the heat-hardenable chromiumnickel stainless steel, particularly the chromium-nickelaluminum stainless steels, and more especially to an art of stabilizing the steels for shipment and then subsequently hardening the same for use. As well, it relates to the stabilized steel and to the hardened steel.

One of the objects of my invention is the provision of an art for stabilizing the heat-hardenable chromiumnickel stainless steels, this in such manner as to reliably assure a freedom from unwarranted hardening in shipping, as in cold weather shipments over substantial distances and over prolonged periods of time.

Another object is the provision of a stabilized heathardenable stainless steel of the character noted which, following shipment, may be variously formed, stamped, punched, spun, drawn, bent, headed, machined and the like, into various articles and products of use, and then hardened at heat treating temperatures sufficiently low as to minimize the formation of heat-tint or other surface discoloration.

,A further object of my invention is the provision of a method for hardening the stabilized heat-hardenable chromium-nickel stainless steels which is simple, effective,

and thoroughly reliable,.and which employs available heat exchange apparatus and equipment.

Other objects of my invention in part will be obvious from a reading of the description which follows and in part particularly pointed out in the course of that description.

h My invention, accordingly, resides in the combination of operational steps, in the temperatures and times of treatment employed, and in the relation of each of the same to one or more of the others as described herein, the scope of the useful application of which is set forth in the claims at the end of this specification.

In order to better understand certain features of my invention it is to be noted at this point that the heathardenable chromium-nickel stainless steels, and especially the chromium-nickel-aluminum stainless steels, have'found great favor in the arts. Attention is drawn to the US. Patent 2,506,558 of May 2, 1950, issued to George N. Goller. And more especially, reference is made to the 17-7PH grade of the Armco Steel Corporation, this: analyzing approximately 16% to 18% chromium, 6% to 8% nickel, 0.50% to 1.50% aluminum, 0.02% to 0.15% carbon, and remainder substantially all iron. Reference also is made to the Armco PH15-7Mo grade analyzing approximately 14% to 16% chromium, 6% to 8% nickel, 0.50% to 1.50% aluminum, 1% to 3% molybdenum, 0.02% to 0.15% carbon, and remainder iron. Additional reference is made to the Armco PH13-7Mo grade which analyzes approximately 12% to 14% chromium, 6% to 8% nickel, 0.50% to 1.50%

aluminum, 3% to 6% molybdenum, 0.02% to 0.15% carbon, and remainder iron.

The various heat-hardenable chromium-nickel stainles's steels noted above are commonly annealed by suitable heat-treatment at the mill, annealing then cooling in air, oil or water, and then shipped to various customer fabricators. The fabricators work and form the metal into a variety of articles of ultimate use and then transform to give a martensite-like constituent and harden the same by desired heat exchange methods employing coldtreatment, followed by heat-treatment to age-harden, or employing a heating and cooling followed by a reheating t0 age-harden, all as noted in said Goller patent.

I find that the heat-hardenable chromiumrnickelaluminum stainless steels are sensitive to particular analysis diiferences so that many of them must be stabilized in order to prevent an unwanted transformation in shiptemperatures as well as in the temperatures of the stabi:

ment as in water weather over long distances. Unfortunately, however, many of these stabilized steels, and many of the steels which, in absence of a special stabilizing treatment, are sufiiciently stable to withstand cold weather shipment without transformation, do not adequately respond to desired transformation and hardening following the working or forming operations.

More particularly, I find that in the 17'7PH stainless steels where chromium is on the low side, or nickel is on the low side, or both chromium and nickel are low, a low annealing temperature makes it diflicult to subsequently stabilize the steel. Conversely, where either or both the chromium and nickel contents are high, the.

steel is stabilized following annealing at low temperature, but, unfortunately, it does not readily respond to cold. temperature transformation and hardening treatment following shipment.

Similarly, the 17-7PH steel, irrespective of vagaries in composition, is sensitive to variations in the annealing lizing treatment. For with an under anneal, that is, anneal at the lower annealing temperatures, there is an inclinationrto transform in cold weather shipmentirrespective of the stabilizing temperatures employed. Moreover, I find that a stabilizing treatment of short duration is ineffective irrespective of the temperature of the stabi lizing treatment.

The various problems encountered greatly multiply when the heat-hardenable chromium-nickel-aluminum. stainless steel is in the form of sheet, strip and other flat' rolled products. Where high annealing temperatures are. employed, there is some inclination to develop a rough surface. And although sheet and strip is stabilized by the" high temperature annealing treatment followed by the stabilizing treatment I find that it frequently is difficult to transform such sheet and strip through the prior art low temperature transformation treatment, that is,.tr eatment at a temperature of about F. V I

Accordingly, one of the objects of my invention istliej Patented Nov. 1, 1960 provision of an art or method of treating the heat-hardenable chromium-nickel stainless steels, and particularly the chromium-nickel-aluminum stainless steels, to positively assure freedom from unwanted transformation in cold weather shipment, irrespective of differences in the chromium and nickel analysis of the various heats of metal being shipped, and yet which fully respond to low temperature transformatiton treatment and heat-hardening treatment to achieve high yield strengths, that is, on the order of 190,000 p.s.i., and ultimate strengths which may exceed 200,000 p.s.i.

Referring now more particularly to the practice of my invention, I first anneal the heat hardenable chromiumnickel stainless steels at a temperature of about 1800 F. This annealing treatment, however, may be conducted at a temperature of 1700 to 1850 F. In general, the lower annealing temperature, that is, on the order of 1700 F., is employed for the analyses of the greater austenite stability, that is, chromium on the high side or nickel on the high side, or both chromium and nickel high. The higher annealing temperatures, that is, about 1850 F., are employed for the steels of the less stability, that is, with chromium on the low side, or nickel on the low side, or both chromium and nickel low. And the time of the annealing treatment which I prefer ranges from a fraction of a minute up to 30 minutes for sheet, strip, light plate and tubes, and some 15 minutes to 1 hour for bars, heavy plate and shapes. The annealed steel is cooled in air, oil or water, as is well known.

I find that the annealing treatment at about 1800" F. gives the austenite constituent of the steel a medium range of stability. While some of the steels in the annealed condition would not transform in cold weather shipment, I find that many would, particularly where temperatures as low as -30 F. are reached. In order to assure a freedom from unwanted transformation at such \low shipping temperatures I stabilize the steel by reheating it to a temperature of about 900 F. for about 4 hours and cooling. Actually, I find that the stabilizing treatment may be conducted at a temperature of 850 to 950 F.

for a period of from; 2 hours at the higher temperature on up to about 12 hours or more at the lower temperature. This treatment assures a consistent suppression of any tendency for cold weather transformation. There is no change in the microstructure of the steel; under the microscope it looks like austenite.

The annealed and stabilized steel is received by the customer in the stabilized condition. It has good forming and working properties and readily lends itself to a variety of operations such as cutting, bending, machining, punching, stamping, heading and the like. Unfortunately, however, I find that the various worked and formed products cannot be immediately hardened by cold-transformation and heat-hardening methods to consistently develop great strength.

In accordance with my invention I find that by reheating the steel in the customer plant at a temperature of about 1100 F. the steel is activated. And by subsequent heat exchange treatment as by cold treatment followed by further heat treatment great strength is consistently had. While I do not care to be bound by the explanation, I feel that there is a precipitation of some carbides which in some way unknown to me assures subsequent transformation by cold treatment and heathardening by final heat treatment. Apparently, with the precipitation of carbides, principally chromium carbides, there is a depletion of the chromium content of the matrix of the metal. And this lessens the stability; chromium although a well-known ferrite-former, also serves to stabilize austenite and a lowering of the effective chromium content lessens the stability. Perhaps there is also some adjustment of the Ms point. In any event, however, the steel more readily transforms upon subsequent cold treatment. And it hardens to a greater degree upon final hardening treatment.

In general, the activation treatment of my invention may be conducted at a temperature as low as 950 F., this for at least one hour, and as high as 1150 F., this for about 5 minutes or less. At the high temperature I find that the time of treatment must not exceed 30 minutes for otherwise there is a loss in ductility and other physical properties. With an activation temperature of 1000" F. I find that time at such temperature should be about /2 hour to 4 hours. And at 1100 F. for a time of 10 minutes to about 30 minutes.

Best results for the under annealed steels (temperatures of about 1775 F.) are had where the activation treatment is had by heating at 1100 F. for about 10 minutes. This activation treatment also is employed for the steels where annealed at the higher annealing temperatures.

The activated steel now is subjected to refrigeration treatment to effect transformation to give a martensitelike constituent and place the metal in condition for final hardening. Preferably, the metal is cooled at a temperature of about 75 F. for a time of 2 hours or more. Although a temperature of about 75 F. is preferred, I find that some latitude in treating temperature is permissible. In general the temperature may range from 30 F. down to 150 F. And the time of treatment from 1 hour to 24 hours.

In the practice of my invention final hardening is achieved by reheating the steel to a temperature range of 700 to 1150 F. The time for treatment is from a couple of minutes up to about 24 hours. Best results are had with a heat-hardening treatment at 900 to 950 F. for about 1 hour. At this narrow hardening temperature range there is had most rapid ageing and peak strength is achieved. I find that where a lower hardening temperature is employed the properties aresomewhat less favorable. And so, too, where a higher precipitation temperature is used less favorable results are had.

As specific illustration of the practice of my invention I have prepared a series of 17-7PH stainless steels of analysis set forth in Table I below. These various samples, in the form of sheet, were annealed and box stabilized, then held at winter temperatures for substantial periods of time to simulate shipping conditions, then activated, refrigerated and finally precipitation-hardened.

TABLE I Chemical analyses of five specific 17-7PH chromiumm'ckel-aluminum stainless steels Heat 0 Mn P S St Or Ni Al Samples, in the form of sheet, of each of the heats of 17-7PH stainless steel of the analyses indicated above were annealed, some at 1750 F., others at 1775 F. and still others at 1800 F., all for a period of about /2 minute. Following this they were box stabilized at a temperature of 900 F. for a period of 4 hours. During this treatment the samples were slowly heated and slowly cooled to simulate commercial practice. They were then maintained at a temperature of 0 F. for 132 hours, in order to simulate winter shipment. Following this the various samples were activated at 1100 F. for 10 minutes, refrigerated at F. for 24 hours and then hardened by heating at a temperature of 900 F. for 1 hour. The mechanical properties of the various samples both immediately following the simulated cold weather shipping conditions and after activation,

transformation and hardening, are given in Table ll below:

TABLE II Mechanical properties of the under annealed and stabilized samples of 17-7PH stainless steel sheet of analyses of Table 1(A) after simulated cold weather shipment and (B) after final hardening (A) After 132 Hrs, F. (B) After Final Hardening Anneal. Heat Temp. 0.2% Per- 0.2% Per- Y.S., T.S., cent Rn Y.S., T.S., cent; Rs p.s.i. p.s.l. E in p.s.i. p.s.l. E in 1, 750 99, 400 158, 600 14 030.0 198, 500 211, 400 8 46.0 52032.-. 1, 775 71, 400 148, 200 20 B97. 0 215, 000 228, 000 4 48. 0 1, 800 62, 800 144, 400 21 1394.0 216, 600 234, 100 7 48. 0 1, 750 73, 100 144, 200 22 B97. 0 221, 600 236, 500 11 47.0 54510.... 1, 775 67, 900 142, 800 23 B96. 0 208,200 226. 500 9 46. 1, 800 65, 800 141, 500 25 B95. 0 196, 100 215, 000 12 44. 0 1, 750 95, 900 152, 300 14 C23. 0 200, 700 215, 200 6 48.0 44102.-- 1, 775 97, 300 155, 300 15 B97. 0 205,200 220.800 7 47. 5 1, 800 65, 500 148,500 B94. 5 209, 800 229, 100 11 46. 5 1. 750 73, 400 146, 100 21 B96. 0 229, 000 242, 700 6 47.0 54685.-- 1, 775 67. 500 141, 000 20 B95. 5 226, 800 240, 700 0G 48.0 1, 800 64, 500 137, 200 23 B93. 5 218, 900 236, 100 5 47.0 1, 750 112, 500 165, 600 11 C32. 0 212, 000 222, 700 0G 45.0 33829-.-- 1, 775 98, 700 165, 700 11 C32. 0 194, 200 215, 500 10 45.0 1, 800 101, 900 162, 600 12 C25. 0 196, 400 218 000 10 46.0

OG=Samp1e broke outside gauge marks.

Study of Table II reveals that all of the samples of the 17-7PH stainless steel, whether annealed at 1750", -1775 or 1800 F., possessed good ductility following cold Weather simulated shipment in the annealed and box stabilized condition. The 0.2% yield strength ranges from 62,800 psi. for one of the samples annealed at 1800 F. to 112,500 psi. for a sample annealed at 1750 F., with elongation in 2" for these samples ranging from 21% and hardness Rockwell B94.0 down to 11% and hardness Rockwell C32.0. At the same time it is to be noted that minimum yield and maximum elongation generally are had with the higher annealing temperatures. In all of the samples the mechanical properties suggest reasonably good workability.

After final hardening it will be seen from Table II that there is had great tensile strength, this in every case exceeding 200,000 p.s.i., with 0.2% yield strengths in every case exceeding 190,000 p.s.i. The hardness in every case is Rockwell C44.0 or better.

The activation treatment employed in the art of my invention is rather critical in terms of the treating temperature employed and the time of treatment. I generally find that with low treating temperatures the mechanical properties following transformation and final hardening are somewhat erratic. Samples activated at 1000 and 1100 F., however, consistently responded to the final hardening treatment, the 1000 F. activation treatment being best maintained for a period of 10 minutes to 4 hours, while the 1100 F. treatment being best for a period of 10 minutes or less to about minutes. All of this is rather pointedly shown in tests made on a representative heat of steel of the Table I, as shown in Table III below. The samples of the steel were annealed at 1800 F. for /2 minute and air cooled, then box stabilized at 900 F. for 4 hours. They were then subjected to simulated cold weather transportation of 336 hours at 0 F., following which they were activated at the temperatures and the times indicated in the table,

TABLE III Efiecl of activation temperatures and times of activation 0n the physical properties of the 17-7PH' stainless steels Activation Heat 52032 0.2% T S Percent;- Temp, 0 F. Time Y.S p s i E in 2' R,

p.s.i.

10 min. 196, 500 12 41. 0 30 min. 180, 300 14 40. 0 900 90 min. 162, 500 14 36.0 4 hrs. 180, 000 12 41. 5 8 hrs 195, 500 10 41. 5 10 min 202, 000 12 41. 0 30 min 206, 000 12 44. 0 1000 90 min 213, mo 10 43. 5 4 hrs 209, 500 10 43. 0 8 hrs 196, 000 12 45. 5 10 min. 217, 000 10 45. 5 30 min 217, 500 4 41. 5 1100 90 min 206, 000 0G 14. 0 4 hrs 170, 50 194, 000 0G 41. 0 5 5 8 hrs 156, 700 178,000 4 38 OG=Sample broke outside of gauge marks.

Thus it will be seen that I provide in my invention an art for consistently and reliably hardening: the chromiumnickel heat-hardenable stainless steels from a fully stabilized condition. Additionally, it will be seen that I provide a steel which is stable, well calculated to resist transformation in winter shipment where sub-zero temperatures are reached and Where the time of shipment may run into many days, and yet which by my novel method of treatment may be hardened to develop yield strengths in excess of 190,000 p.s.i. and ultimate strengths in excess of 200,000 psi.

While best results in the art and products of my invention are had in the heat-hardenable chromium-nickelalurninum stainless steels, I find that many beneficial characteristics are had with the treatment of any heathardenable chromium-nickel stainless steel. Benefit notably is bad in the heat-hardenable chromium-nickel stainless steels containing .5 to 8.0% of molybdenum or tungsten or both, or copper in the amount of .5% to 4.0%.

In the art and products of my invention there are enjoyed many new and unsuspected beneficial characteristics hereinbefore set forth, together with many thoroughly practical advantages. Since many embodiments may be made of the art of my invention and since many changes may be made in the embodiments herein particu:

larly set forth, it is to be understood that all matter described in this specification is to be interpreted as illustrative and not by way of limitation.

I claim as my invention:

1. In the production of heat-hardened chromium-nickel stainless steel products, the art which comprises annealing a heat hardenable chromium-nickel stainless steel at a temperature of 1700 to 1850 F. and cooling; preheating the steel at a temperature of 850 to 950 F. to stabilize the same forming the steel into products of desired configuration; activating the stabilized steel by reheating at a temperature of 950 to 1150 F. for a time of about 5 minutes or less at the higher temperature to 1 hour or more at the lower temperature; transforming the steel by refrigeration at a temperature of 30 to l50 F.; and then hardening the activated and refrigerated steel by reheating at a temperature of 700 to 1150 F.

2. In the production of heat-hardened chromium-nickelaluminum stainless steel sheet and strip products, the art which comprises annealing heat-hardenable chromiumnickel-aluminum strip at a temperature of 17 to 1850 F. for a fraction of 1 minute to 10 minutes and cooling; stabilizing the same by reheating at a temperature of 850 to 950 F.; working or fabricating the sheet or strip into desired products; activating the steel by reheating at a temperature of 950 to 1150 F. for a time of about 5 minutes or less at the higher temperature and 1 hour or more at the lower temperature; transforming the steel by refrigerating the same at a temperature of about -75 F.; and then hardening the same by reheating the products at a temperature of about 900 F.

3. In the production of heat-hardened chromium-nickelaluminum stainless steel bar products, the art which comprises annealing heat-hardenable chr0mium-nickelaluminum stainless steel bar stock at a temperature of 8 l700 to 1850 F. for about 15 minutes to 1 hour and cooling; stabilizing the same by reheating at a temperature of 850 to 950 F. for 12 hours or more at 850 to about 2 hours for 950 F.; working or forming the steel into desired products; activating the same by heating at a temperature of 950 to 1150 F., the heating at the higher temperature being not over 30 minutes, while that atthe lower temperatur e being at least 1 hour; transforming the same by refrigeration at a temperature of about -75 F; and hardening the products by reheating at a temperature of about 900 4. In the production of hardened chromium-nickel stainless steel products, the art which comprises activating previously annealed and stabilized heat-hardenable chromium-nickel stainless steel products by heating the same at a temperature of 950 to 1150 F. for a time of about 5 minutes or less at the higher temperature to 1 hour or more at the lower temperature; transforming the same by refrigeration at a temperature of 30 to -l F.; and hardening the same by reheating at a temperature of 750 to 1150 F.

5. In the production of hardened chromium-nickelaluminum stainless steel products, the art which comprises activating previously annealed and stabilized heat-hardenable chromium-nickel-aluminum stainless steel products by heating the same at a temperature of about 1100 F. for about 10 to 30 minutes; transforming the same by refrigeration at a temperature of about F.; and hardening the same by reheating at a temperature of about 900 F.

References Cited in the file of this patent UNITED STATES PATENTS Goller May 2, 1950 Lena July 16, 1957 OTHER REFERENCES 

1. IN THE PRODUCTION OF HEAT-HARDENED CHROMIUM-NICKEL STAINLESS STEEL PRODUCTS, THE ART WHICH COMPRISES ANNEALING A HEAT-HARDENABLE CHROMIUM-NICKEL STAINLESS STEEL AT A TEMPERATURE OF 1700* TO 1850*F. AND COOLING, PREHEATING THE STEEL AT A TEMPERATURE OF 850* TO 950*F. TO STABILIZE THE SAME FORMING THE STEEL INTO PRODUCTS OF DESIRED CONFIGURATION, ACTIVATING THE STABILIZED STEEL BY REHEATING AT A TEMPERATURE OF 950* TO 1150*F. FOR A TIME OF ABOUT 5 MINUTES OR LESS AT THE HIGHER TEMPERATURE TO 1 HOUR OR MORE AT THE LOWER TEMPERATURE, TRANSFORMING THE STEEL BY REFRIGERATIN AT A TEMPERATURE OF -30* TO -150*F., AND THEN HARDENING THE ACTIVATED AND REFRIGERATED STEEL BY REHEATING AT A TEMPERATURE OF 700* TO 1150*F. 